1. Field of the Invention
The present invention relates to a drive circuit for a fluorescent display.
2. Description of the Related Art
A drive circuit 10 for a conventional fluorescent display 12 is illustrated schematically in FIG. 4. The fluorescent display 12 includes a filament F, a plurality of anode electrodes A1-An, and a plurality of grids G1-Gm. The filament F emits thermoelectrons into a vacuum case, not shown. Each of the anodes A1-An are covered with a fluorescent substance. The grids G1-Gm control the thermoelectrons emitted from the filament F. A control circuit 11 selectively applies a first direct current (DC) input voltage HV1 to the anodes A1-An through anode lines a1-an. The control circuit 11 also selectively applies a second DC input voltage HV2 to the grids G1-Gm through grid lines g1-gm. A third DC input voltage HV3 is applied to the filament F through a first switching element Tr1. A first pulsed control voltage Vc1 is applied to the first switching element Tr1.
Generally, the same voltage is used for each of the input voltages HV1, HV2, HV3. The first switching element Tr1 is switched ON and OFF by the pulsed control voltage Vc1, to chop the third input voltage HV3 into a pulsed DC voltage that is applied to the filament F.
The anodes A1-An are arranged opposed to the grids G1-Gm. When an opposed anode A1 and grid G1 are selected at the same time (i.e., when input voltages HV1 and HV2 are simultaneously applied to A1 and G1), the fluorescent substance on the anode A1 emits light. However, the fluorescent substance covering the non-selected anodes A2-An may also emit a slight amount of light due to leakage emission. To prevent leakage emission, the drive circuit 10 applies a cutoff bias voltage to the non-selected anodes A2-An and/or grids G2-Gm. To generate the cutoff bias voltage, a Zener diode ZD is used in the conventional fluorescent display 12. See, e.g., Japanese patent publication Nos. 02-190893 and No. 2007-72323, the contents of which are fully incorporated herein.
In operation, when the first switching element Tr1 is turned ON, a filament current flows into the filament F. The filament current also flows through the Zener diode ZD and generates reactive power therein. For this reason, a second switching element Tr2 is connected to the Zener diode ZD in parallel. The second switching element Tr2 is controlled by a second pulsed control voltage Vc2 from the control circuit 11 and can be switched ON and OFF. When the filament current flows into the filament F (i.e., while the first switching element Tr1 is ON), the second switching element Tr2 is turned ON by the pulsed control voltage Vc2 and the Zener diode ZD is shorted.
Therefore, when the filament current flows through the filament F, no current flows through the Zener diode ZD and as a result, no reactive power is generated. The drive circuit 10 is needed to connect the second switching element Tr2 to the Zener diode ZD. However, ON-OFF control of the second switching element Tr2 needs to be coordinated with the first switching element Tr1. As a result, the control of the second switching element Tr2 becomes complex.
In view of the above problem, it would be desirable to have a drive circuit for a fluorescent display in which the filament current does not flow through the Zener diode used to generate the cutoff bias voltage, thus eliminating the need for a switching element connected in parallel to short the Zener diode.